Implement forsyth export

chess/src/main/scala/Game.scala

@@ -1,6 +1,7 @@
 package lila.chess
 
 import format.PgnDump
+import scala.math.max
 
 case class Game(
     board: Board = Board(),
@@ -33,9 +34,23 @@ case class Game(
     orig: Pos,
     dest: Pos,
     promotion: PromotableRole = Queen): Valid[Game] =
-      apply(orig, dest, promotion) map (_._1)
+    apply(orig, dest, promotion) map (_._1)
 
   lazy val situation = Situation(board, player)
 
   def pgnMovesList = pgnMoves.split(' ').toList
+
+  /**
+   * Halfmove clock: This is the number of halfmoves
+   * since the last pawn advance or capture.
+   * This is used to determine if a draw
+   * can be claimed under the fifty-move rule.
+   */
+  def halfMoveClock: Int = board.history.positionHashes.size
+
+  /**
+   * Fullmove number: The number of the full move.
+   * It starts at 1, and is incremented after Black's move.
+   */
+  def fullMoveNumber: Int = 1 + turns / 2
 }

chess/src/main/scala/format/Forsyth.scala

@@ -13,5 +13,35 @@ object Forsyth extends Format[Game] {
     Game()
   }
 
-  def >>(game: Game): String = ""
+  def >>(game: Game): String = List(
+    exportBoard(game.board),
+    game.player.letter,
+    game.board.history.castleNotation,
+    ((for {
+      lastMove ← game.board.history.lastMove
+      (orig, dest) = lastMove
+      piece ← game board dest
+      if piece is Pawn
+      pos ← if (orig.y == 2 && dest.y == 4) dest.down
+      else if (orig.y == 7 && dest.y == 5) dest.up
+      else None
+    } yield pos.toString) getOrElse "-"),
+    game.halfMoveClock,
+    game.fullMoveNumber
+  ) mkString " "
+
+  private def exportBoard(board: Board) = {
+    {
+      for (y ← 8 to 1 by -1) yield {
+        (1 to 8).map(board(_, y)).foldLeft(("", 0)) {
+          case ((out, empty), None)        ⇒ (out, empty + 1)
+          case ((out, 0), Some(piece))     ⇒ (out + piece.forsyth.toString, 0)
+          case ((out, empty), Some(piece)) ⇒ (out + empty.toString + piece.forsyth, 0)
+        } match {
+          case (out, 0)     ⇒ out
+          case (out, empty) ⇒ out + empty
+        }
+      } mkString
+    } mkString "/"
+  } mkString
 }